During the past year, we investigated OCT (optical coherence tomography) imaging as functional measurements of retinal activity. OCT has been widely used as a non-invasive tool to provide optical sections of biological tissues. Because the optics of the eye allows for high resolution of retinal structure, OCT is often used in the diagnosis and monitoring of retinal disease progression in the clinic and the laboratory. As OCT technique uses infrared light in generating image, it also provides a useful tool to study light-induced changes in the retinal structure and thickness. OCT images obtained from mice in room light exhibit a prominent dark band between the RPE and photoreceptor tip layers when compared with those obtained from the same eyes after over-night dark-adaptation. There was a significant increase (6.8 +/- 0.7 micron, n=13) in total retina thickness for light-adapted OCT images compared with the image from the same eye under dark-adapted condition. These light-adapted retinal thickness changes were manifested mainly in the outer retina, with an additional hypo-reflective band between the RPE and photoreceptor-tip layers observed only on light-adapted OCT images. Elongation of the outer retina be detected even with brief light exposures, increasing 1.5 +/- 0.2 micron after 15 min (n=3), and 4.1 +/- 1.0 micron after 2 hr (n=6). Conversely, the outer retina shortening could be detected of 1.5 +/- 0.3 micron (n=3) and 3.0 +/- 0.5 micron (n=8) after 15 min and 2 hr dark adaptation, respectively. Histological analysis revealed similar changes in distal portion of the mouse retina, with an elongation between the outer limiting membrane and Bruchs membrane from 45.8 +/- 1.7 micron (n=5) of dark-adapted retinas to 52.1 +/- 3.7 micron (n=5) of light-adapted retinas. Light adapted retinas also exhibited a dense staining of RPE apical microvilli with phalloidin at the same location as the new hypo-reflective band observed on the OCT images. In conclusion, light-adaptation leads to an elongation in outer retinal regions from outer limiting membrane (OLM) to retinal pigment epithelium (RPE) cells which can be detected by both OCT image and in histological sections in the mouse retina. Conversely, dark-adaptation induces a contraction in the same region of the retina. The response takes minutes to develop and hours to saturate. These adaptational changes may need to be considered by automated OCT clinical segmentation programs used to determine the retinal layer thickness, and to identify retinal disease progression. It is likely that the changes in sub-retinal fluid regulated, in large part, by RPE and photoreceptor cell activity. Therefore, light-evoked OCT response could be used as a non-invasive metric of RPE cell fluid transport function in the intact eyes.